1. Field of the Invention
This invention relates to an electric fluid heater and, more particularly, to a demand-type electric heater in which the fluid is heated on demand only and the output temperature is maintained at a continuous temperature over wide ranges of flow rates.
The present invention has wide applicability for home installations as well as factory, laboratory and portable electric heaters for special purpose applications. The invention will be described in connection with a water heater having applicability for a home installation since many of the problems presented by a home hot water heater are solved by the instant invention. It is expressly understood, however, that the invention is not limited to the heating of water nor is it limited to home installation.
2. Description of the Prior Art
Instant electric water heaters have the potential benefit and advantage of eliminating the hot water storage tank usually associated with the conventional gas-fired or oil-fired or electric immersion type hot water heater systems. In the usual home hot water system a hot water tank of the order of 40 or 50 gallon capacity and usually glass lined contains an electric, gas or oil-fired heating mechanism which is preset to keep the stored water at a predetermined temperature and available for use by the occupants of the home.
The lowering of temperature of water within the boiler automatically causes the firing system to ignite (i.e., to begin applying heat to the water), thereby supplying more heat to maintain the temperature of the water within the preset limits. Similarly, as the water is used and fresh cold water is inserted into the tank, the water temperature is lowered and again the firing units are ignited for supplying heat to the water.
The instant hot water heaters do not contain a storage tank, but rather are connected intermediate the source of cold water and the ultimate user. A requirement for hot water is made by turning the hot water faucet into the "on" position, thereby allowing the cold water to pass through the heating chamber of the instant hot water system and out the hot water faucet for use by the ultimate user. Electric current is caused to pass through the water within the heating chamber of the electric hot water system, thereby instantly heating the water which is supplied to the user as demanded and, hence, the term "demand system."
The instant hot water systems should be differentiated from the so-called electric immersion water heaters which normally contain resistive elements which are inserted into a water tank or similar container of water where the heat generated by the passage of current through the resistive element is transferred to the water in a heat transfer relationship. These electric immersion water heaters are conventionally used for home hot water heaters and are also used to heat small quantities of water, such as in portable containers or individual cup-sized containers for the making of beverages requiring hot water.
This invention is concerned with an instant water heating system in which the water is passed through a chamber containing electrodes and electric current is passed through the spaced-apart electrodes and through the fluid, thereby instantly heating the fluid for use by the ultimate consumer.
Instant hot water heaters of the type utilizing immersed current-conducting electrodes are known in the prior art and have been fully described in the Grupp U.S. Pat. No. 2,529,688 and in the Mamoulides U.S. Pat. No. 3,513,281.
The recognized problems associated with instant hot water heaters results from the fact that the electric current does not flow until the water is flowing through the heating chamber as evidenced by a demand for hot water. The rate of heat transfer between the electric current and the water is therefore very high, and it is imperative that automatic temperature control means be provided to regulate the water temperature during the heating process.
The problem is compounded by the fact that water varies in conductivity from location to location as a function of impurities and salts located within the water. So-called "hard water" contains concentrations of calcium, magnesium and iron in varying degrees, resulting in the hardness of the water. The parts per million (p.p.m.) of the dissolved minerals in the water is a measure of the hard water which is known to vary over a range of 10 p.p.m. to 1400 p.p.m.
In addition, the salinity of the water also varies the conductivity of the water, and it is known that the electrical conductivity of the water is proportional to the percentage of dissolved materials regardless of whether the water is of the hard type or soft type. The conductivity of water is generally expressed in terms of micro Mhos per cubic centimeter ( .mu.mho/cm.sup.3). Conductivity of municipal water varies enormously. Municipal water having conductivity as high as 1,660 .mu.mho/cm.sup.3 at 25.degree. centigrade is known in the United States and even higher conductivities may exist.
The wide range of conductivity of the water has severely limited the development of instant demand electric hot water heater systems since the variations of the resistivity of the water of only one-half an ohm is a 220-volt system can vary the demand for current from 220 amperes to 440 amperes. In addition, conductive particles flowing through the water can cause instantaneous shorts capable of destroying the equipment and causing great damage.
The prior art has recognized these problems and has attempted to compensate for the changing conductivity of the water by utilizing mechanical devices for moving the spaced-apart electrodes within the heating chamber into a controllable relationship from each other in an attempt to maintain the same resistivity load between the electrodes. The Mamoulides patent mentioned above describes external mechanical means for physically varying the spacing between the spaced-apart electrodes.
It is envisioned that the operator would increase the spacing between the spaced-apart electrodes in those areas where the water is highly conductive and conversely reduce the spacing between the electrodes where the water has less conductivity. Unfortunately, such prior art devices cannot adapt to instantaneous changes or other temporary changes in the conductivity or hardness of the water.
In order to minimize the shock hazard associated with electric hot water heaters, it is necessary that the unit be grounded and that the leakage current between the electrodes and the ground connection be controlled within certain limits as determined by the Underwriters Laboratories. In the usual installation, the cold water pipes feeding the electric hot water heater are at ground potential and the water flowing past the electrodes in the heating chamber determine the leakage path between the electrodes and the ground. Changing conductivity of the water due to harness or immersed salts or any other reason will also affect the leakage current passing from the electrodes to ground, which current can create other dangerous conditions. The prior art has not disclosed how to handle the changing leakage current as a result of the changing conductivity of the fluid medium.